scsi: qla2xxx: Fix error handling in qlt_alloc_qfull_cmd()

The test "if (!cmd)" is not useful because it is guaranteed that cmd !=
NULL.  Instead of testing the cmd pointer, rely on the tag to decide
whether or not command allocation failed.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Fixes: 33e79977 ("qla2xxx: Add support for QFull throttling and Term Exchange retry") # v3.18.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>